This application claims the priority of German patent 197 20 453.8, filed May, 15, 1997, in Germany, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a method and device for obtaining nitrogen by low-temperature separation of air by two-stage rectification in a double column. The double column has a high-pressure column and a medium-pressure column that are in a heat-exchange relationship with each other. In the process, air is compressed, purified, and cooled in a main heat exchanger against separation products, and fed to rectification. At least one nitrogen product fraction is removed from the high-pressure column and a nitrogen gas fraction from the double column is heated, expanded, and at least partially brought into indirect heat exchange with an oxygen-enriched liquid from the lower region of the medium-pressure column. The nitrogen gas fraction is at least partially condensed and the oxygen-enriched liquid is at least partially evaporated, and the condensate formed in the indirect heat exchange is at least partially recycled to the medium-pressure column.
The basic information for low-temperature separation of air in general and the construction of double-column systems in particular is known from the monograph entitled "Tieftemperaturtechnik" ("Low-Temperature Technology") by Hausen/Linde (Second Edition, 1985) or from a paper by Latimer in Chemical Engineering Progress (Vol. 63, No. 2, 1967, p. 35). The heat-exchange relationship between the high-pressure column and the medium-pressure column of a double column is generally brought about in a main condenser, in which head gas in the high-pressure column is liquefied against evaporating bottom liquid in the medium-pressure column.
A method of this type is disclosed in DE 4441920 Cl. In this reference, the nitrogen gas fraction is formed by head gas in the medium-pressure column. Before it is expanded, it is first heated to ambient temperature then compressed and cooled again. This method is not economically satisfactory in all cases.
Hence the goal of the present invention is to provide a method and corresponding device for the separation of air having especially high economy, particularly through low energy consumption and/or low equipment cost.
This goal is achieved by heating the nitrogen gas fraction upstream of expansion to an intermediate temperature between the temperatures of the cold end and the warm end of the main heat exchanger.
Heating of the nitrogen gas fraction is usually effected by indirect heat exchange. It can occur for example in the main heat exchanger used to cool entering air. According to the present invention, heating of the nitrogen gas fraction from the intermediate temperature to the temperature at the warm end of the main heat exchanger (usually approximately the same as ambient temperature) and the corresponding re-cooling are at least partially unnecessary. Exchange losses in the corresponding heat exchanger are correspondingly lower, i.e. less energy is lost by irreversibilities. The corresponding heat exchanger can also have fewer passages and accordingly can be manufactured more economically.
The intermediate temperature to which the nitrogen gas fraction is heated is, for example, 140 to 190 K less than the temperature of the warm end of the main heat exchanger. Expansion of the nitrogen gas fraction upstream of its condensation by indirect heat exchange preferably leads to a pressure intermediate between the pressures of the high-pressure column and the medium-pressure column or to a pressure below the pressure of the medium-pressure column. Accordingly, the pressure of the condensate before it is fed into the medium-pressure column has to be decreased or increased, for example, by a throttle valve or a pump.
Preferably, the nitrogen gas fraction is not cooled between being heated to an intermediate temperature and being expanded. Hence, there are no irreversibilities due to heating and re-cooling of the nitrogen gas fraction and the corresponding heat exchange devices.
Moreover, it is preferable for the nitrogen gas fraction to come from the high-pressure column. In this case in particular, the nitrogen gas fraction does not need to be compressed between heating to the intermediate temperature and expansion, so that the corresponding machine and its associated energy consumption are unnecessary.
If some of the condensate formed in the indirect heat exchange is fed to the high-pressure column, the corresponding quantity can additionally be obtained as a high-pressure product from the high-pressure column. To increase the pressure in the condensate to the high-pressure column level, a pump for example may be used.
Any known method may be used to expand the nitrogen gas fraction, but this expansion is preferably effected in a work-producing manner, for example in a turbine. In this way, some or all of the cold needed for the process can be obtained. Additionally, it is possible to use the energy obtained from expansion at least partially to compress a product stream, for example by mechanical coupling of the expansion machine to a compressor.
Particularly for the case that the expanded quantity of nitrogen gas fraction is so large that it cannot be completely liquefied against the oxygen-enriched liquid, it is favorable for some of the expanded nitrogen gas fraction to be condensed in indirect heat exchange with an intermediate liquid from the medium-pressure column. This avoids raising the input pressure when the nitrogen gas fraction is expanded, even with a relatively high cold requirement. The condensate arising from this indirect heat exchange is preferably fed to the medium-pressure column. The gas produced when the intermediate liquid is evaporated is preferably recycled into the medium-pressure column. The addition heating of the medium-pressure column thus produced improves the separating effect of this column. The corresponding additional condenser-evaporator can be located inside or outside the medium-pressure column.
The intermediate liquid is preferably drawn off in an region below the point at which sump liquid is fed to the high-pressure column, and at least one, preferably 1 to 30, for example 20, theoretical levels or trays above the medium-pressure column sump.